The present invention relates generally to fluid valves, and more particularly to a bleed valve for a gas turbine engine.
There are a variety of know gas turbine engine configurations, including two-spool, high-bypass engines. Such an engine typically includes a high pressure spool, a combustion system and a low pressure spool disposed within an engine case to form a generally axial, serial flow path about the engine centerline. The high pressure spool includes a high pressure turbine, a high pressure shaft extending axially forward from the high pressure turbine, and a high pressure compressor connected to a forward end of the high pressure shaft. The low pressure spool includes a low pressure turbine, which is disposed downstream of the high pressure turbine, a low pressure shaft, which typically extends coaxially through the high pressure shaft, and a low pressure compressor connected to a forward end of the low pressure shaft, forward of the high pressure compressor. A fan is also connected to the low pressure spool, forward of the low pressure compressor. The combustion system is disposed between the high pressure compressor and the high pressure turbine and receives compressed air from the compressors and fuel provided by a fuel injection system. A combustion process is carried out within the combustion system to produce high energy exhaust gases to produce thrust and turn the high and low pressure turbines, which drive their respective compressors and the fan to sustain the combustion process.
The high and low pressure spools are efficient when interacting with gases, in part, due to the seals between the static and the rotating components of the spools. Therefore, a substantial amount of energy is required to rotate the spools, and that energy is normally provided by the high energy gases from the combustion system. However, prior to starting the engine, these gasses are not available for use, so a starter motor must be used to turn one or both of the spools. In order to decrease the load on the starter, one or both of the compressors can be bled or essentially bypassed by employing a start bleed valve. During start, a compressor bleed valve is open and allows gas from the compressor to exit the engine without being further compressed or traveling through the combustion system or the turbines. Once the engine is started and reaches idle speed, the compressor bleed valve is typically closed, although it can be actuated in specific operational conditions to maintain compressor stability and prevent compressor stall or surge. A bleed valve that can be actuated to both aid in engine starting and provide engine compressor stability is known as a start and stability bleed valve. Unfortunately, the icy ambient conditions that a gas turbine engine can encounter during normal operation can freeze a traditional start or stability bleed actuation system, preventing it, for example, to open. If the valve fails to actuate open, it can prevent starting the engine and/or prevent proper compressor stability control.